Data Link Control
Minimum 2 devices are needed for data
communication. So line discipline is necessary
for co-operation b/w2 devices.
The 2 important functions of data link layer
is flow control and error control.This
functions are otherwise called as Data link
control.
Communication

Line Discipline
It coordinates the link system
It is done in 2 ways
ENQ (Enquiry)
Used in peer peer communication
Enquire whether there is arequired link b/w
two devices
Check whether the intended device is capable to receive
ACK (Acknowledgment)
Used in Primary secondary communication
The intended device willacknowledge about its status to the
receiver

There are2 categoriesinline
discipline

Select
It is a line discipline used in topologies
with primary secondary relationship.
Select
It is uses whenever the primary device
has something to send.ie)Primary
controls the link.

Poll
The polling function is used by the
primary device to Select transmissions
from the secondary devices.
If the primary device is ready to receive
data , It ask each device in turn if it has
anything to send.

Flow Control
It is a set of procedures to tell the sender
how much data it can transmit before it must
wait for an acknowledgement from the
receiver.
Two categories of flow control:
Stop-and-wait
Send one frame at a time.
Sliding window
Send several frames at a time.

Data Link layer Presentation form Behrouz and furozen

Stop-and-wait
Sender sends one frame and waits for an
acknowledgement before sending the next frame.

Stop-and-wait
Advantages:
Simplicity
.
Each frame is checked and acknowledged
before the next frame is sent.
Disadvantages:
Slow.
Can add significantly to the total transmission time
ifthe distance betweendevices is long.
Inefficiency
Each frame is alone on the line.

SlidingWindow
Sender can send several frames before
needing an acknowledgement.
Advantages:
The link can carry several frames at once.
Its capacity can be used efficiently.

Can detect all single-bit errors. Can detect burst errors only if the total
number of errors in each data unit is odd.
Even number of ones
add 0
Odd number of ones
add 1

Increases the likelihood of detecting burst errors.
n bits LRC can detect a burst error of n bits.
Errors may be undetected if:
Have even number of errors in that position.

You will experience a painful
sharpening from time to time, but
this is required if you are to become
a better pencil.

You have the
ability to
correct any
mistakes you
might make.

Hamming Codes-Error correction
Hamming codes, like polynomial codes,
are appended to the transmitted message
Hamming codes, unlike polynomial codes,
contain the information necessary to
locate asingle bit error

CalculatingtheHamming Code
The key to the Hamming Code is the use of extra parity bits to allow the
identificationof asingle error. Create the code word as follows:
Mark allbit positions that are powers of two as parity bits.(positions 1,2,4,8, 16,
32, 64,etc.)
All other bit positions are for the data to be encoded. (positions 3,5,6,7,9,10,
11,12,13,14,15,17,etc.)
Each parity bit calculatesthe parity for some of the bits in the code word.The
position of the parity bit determines the sequence of bits that it alternately
checks and skips.
Position 1:check 1 bit,skip 1 bit,check 1 bit,skip 1 bit,etc.(1,3,5,7,9,11,13,15,...)
Position 2:check 2 bits,skip 2 bits,check 2 bits,skip 2 bits,etc.
(2,3,6,7,10,11,14,15,...)
Position 4:check 4 bits,skip 4 bits,check 4 bits,skip 4 bits,etc.
(4,5,6,7,12,13,14,15,20,21,22,23,...)
Position 8:check 8 bits,skip 8 bits,check 8 bits,skip 8bits,etc.(8-15,24-31,40-
47,...)
Position 16:check 16 bits,skip 16 bits,check 16 bits,skip 16 bits,etc.(16-31,48-
63,80-95,...)
Position 32:check 32 bits,skip 32 bits,check 32 bits,skip 32 bits,etc.(32-63,96-
127,160-191,...)
etc.
Set aparity bit to 1 ifthe totalnumber of ones in the positions it checks is odd.
Set aparity bit to 0 ifthe total number of ones in the positions itchecks is even.

Position ofRedundancy bitin
Hamming code

MAC
IEEE has subdivided(Project 802) the data
link layer into two sub layers:
Logical Link Control
Medium access control
Functions of MAC
It resolves the contention of shared media
It contains allinformation to move information
from one placeto another
It contains the physical address of next station
to route packet.
MAC protocol are specific to LAN

The project 802 which governs internet working. Here each
subdivision is identified by a number
802.1(internetworking)
802.2(LLC)
and MAC modules
802.3(CSMA/CD)
802.4(Tokenbus)
802.5(Tokenring)

Figure 13.1 IEEE standard for LANs

MAC protocol arespecifictoLAN
LAN is aLocal Area Network used for
communication inside building
Protocols for LAN are,
Ethernet
Token Ring
Token bus
FDDI

IEEE STANDARDS
Ethernet: It is a LAN protocol that is used in Bus and Star topologiesand
implements CSMA/CD as the medium access method
Original (traditional) Ethernet developed in 1980 by
three companies: Digital, Intel, Xerox (DIX).
In 1985, the Computer Society of the IEEE started a
project, called Project 802, to set standards to enable
intercommunication among equipment from a variety of
manufacturers.
Current version is called IEEE Ethernet

IEEE 802.3 supports LAN standardEthernet
IEEE802.3 defines two categories
Baseband
Broadband
Base band has fivedifferent category
10Base5
10Base2
10BaseT
1Base5 etc.,
Broad band has a category
10Broad36

Access Method:CSMA/CD
When multiple user access the single line ,there is a
danger of signals overlapping and destroying each
other(Traffic) .such anoverlap is called Collisions.
To avoid this the access method used in Ethernet is
carrier sense multiple access/collision detection
In CSMA any workstation wishing to transmit must
listen to existing traffic on the line
If no voltage is detected,lineis considered idle
CSMA cuts down the number of collisions, but cant
eliminate. Collisions still occur if both station try to
listen ata time.

IEEE Ethernet
In IEEE 802.3 Ethernet Data link layeris splitinto two sublayers:
Bottom part:MAC
The frameis called IEEE 802.3
Handles framing,MAC addressing,MediumAccess control
Specific implementation for each LAN protocol
Defines CSMA/CD as the access method for Ethernet LANs
and Token passing
method for Token Ring.
Implemented in hardware
Top part:LLC (Logical Link Control)
The subframe is called IEEE 802.2
Provides error and flow control ifneeded
It makes the MAC sublayer transparent
Allows interconnectivitybetween different LANs data link layers
Used to multiplex multiplenetwork layer protocols in the datalink
layer
frame
Implemented in software

Ethernet Provides Unreliable, connectionless Service
Ethernet data link layer protocol provides
connectionless service to the network layer
No handshaking between sending and receiving
adapter.
Ethernet protocol provides Unreliable service to the
network layer :
Receiving adapter doesn t send ACK or NAK to
sending adapter
This means stream of datagrams passed to network
layer can have gaps (missing data)
Gaps will be filled if application is using reliable transport layer
protocol
Otherwise, application will see the gaps

Ethernet
Frame formats. (a) DIX Ethernet ,(b) IEEE
802.3.
FCS
FCS
Ethernet Frame format

8 bytes with pattern 10101010 used to synchronize receiver, sender clock rates.
In IEEE 802.3, eighth byte is start of frame (10101011)
Addresses: 6 bytes (explained latter)
Type (DIX)
Indicates the type of the Network layer protocol being carried in the payload
(data) field, mostly IP but others may be supported such as IP (0800), Novell IPX
(8137) andAppleTalk (809B),ARP (0806) )
Allow multiple network layer protocols to be supported on a single machine
(multiplexing)
Its value starts at 0600h (=1536 in decimal)
Length (IEEE 802.3): number of bytes in the data field.
Maximum 1500 bytes (= 05DCh)
CRC: checked at receiver, if error is detected, the frame is discarded
CRC-32
Data: carries data encapsulated from the upper-layer protocols
Pad: Zeros are added to the data field to make the minimum data length = 46 bytes
Ethernet Frame
PREAMBLE

Ethernet address
Six bytes = 48 bits
Flat address not hierarchical
Burned into the NIC ROM
First three bytes from left specify the vendor. Cisco 00-00-
0C, 3Com 02-60-8C and the last 24 bit should be created
uniquely by the company
Destination Address canbe:
Unicast: second digit from left is even (one recipient)
Multicast: Second digit from left is odd (group of stations
to receive the frame conferencing applications)
Broadcast (ALL ones) (all stations receive theframe)
Source address is always Unicast

Figure 13.3 Ethernet evolution through four generations

Categories of traditional Ethernet
<data rate><Signaling method><Max segment length or cable type>

IEEE 802.3 Cable Types
Name Cable Max. Max Cable
Segment
Length
Nodes
/segment
Toplogy
10Base5 thick coax 500 meters 100
Bus
10Base2 thin coax 185 meters 30
Bus
10BaseT twisted pair 100 meters 1 Star
10BaseF Fiber Optic 2Km 1
Star

Figure 13.10 10Base5 implementation

Connection of stations to the medium using 10Base2

10BaseT
Uses twisted pair Cat3 cable
Star-wire topology
A hub functions as a repeater with additional functions
Fewer cable problems, easier to troubleshoot than coax
Cable length at most 100 meters

Figure 13.12 10Base-T implementation

Figure 13.13 10Base-F implementation

Fast Ethernet
100 Mbps transmission rate
same frame format, media access, and collision
detection rules as 10 Mbps Ethernet
can combine 10 Mbps Ethernet and Fast Ethernet
on same network using a switch
media: twisted pair (CAT 5) or fiber optic cable
(no coax)
Star-wire topology
Similar to 10BASE-T
CAT 3
CAT 5

Figure 13.19 Fast Ethernet topology

Figure 13.20 Fast Ethernet implementations

Gigabit Ethernet
Speed 1Gpbs
Minimum frame length is 512 bytes
Operates in full/halfduplex modes mostly
full duplex

In the full-duplex mode of Gigabit Ethernet,
there is no collision;
the maximum length of the cable is
determined by the signal attenuation
in the cable.

Figure 13.23 Gigabit Ethernet implementations

10Gbps Ethernet
Maximum link distances cover 300 m to 40 km
Full-duplex mode only
No CSMA/CD
Uses optical fiber only

TokenRing
It allows each station to sent one frame
.
The access control mechanism used by
Ethernet is inefficient sometimes because
of collision.
It solves the collision problem by passing
token
Initially astation waits for token, ifa
token is free the station maysend adata
frame

Cont..,
This frame proceeds around the ring ,being
regenerated by each station .Eachstation
examines the destination address finds the
frame is addressed to another station and relays
it to its neighbor
.
The intended recipient recognizes its own
address and copies the message and set the
address bit
The token finallyreach the sender and it
recognizes that the datais delivered through
address bit
Token is passed from NIC to NIC

TokenBus
It combines the feature of token ring and
Ethernet

FDDI
Fiber Distributed Data Interface
local area network protocol standardized by ANSI
100-Mbps token passing
Dual-ring LAN
A high-speed backbone technology
High bandwidth
Optical fiber transmission
Allows up to 1000 stations

Components of FDDI
Fiber optic cable
A concentrator (ring)
Stations: 2 types
DAS (Dual Attachment Station) or Class
A:
Connected to both the rings
SAS (Single Attachment Station) or Class
B:
Connected to primary ring

FDDI Frame Format
Similar to token ring
frame

Networking andinternetworking
devices:
An internet is ainterconnection of
individualnetwork. So to create ainternet
we need ainternetworking devices. ie)
Linking anumber of
Internet -WWW
internet-Interconnection of LAN

Why Interconnect?
To separate / connect one corporate division with another.
To connect two LANs with differentprotocols.
To connect a LAN to the Internet.
To break a LAN into segments to relieve traffic congestion.
To provide a security wall between two different types ofusers.

Connecting Devices
Networking Devices
Repeaters Bridges
Internetworking
Devices
Routers Gateways

Introduction
Many times it is necessary to connect a local area network to anotherlocal
area network or to a wide area network.
Local area network to local area network connections are usually performed
with a bridge.
Local area network to wide area network connections are usually performed
with a router.
A third device, the switch, can be used to interconnect segmentsof a local
area network.

Repeater:
A repeater is aregenerator, not an
amplifier
A repeater installed on alink receives the
signal before it becomes too weak or
corrupted ,regenerates the original bit
pattern, and put the refreshed copy back
onto the link.

Gateways:
A gatewayis aprotocolconvertor.
It accepts apacket format for one
protocol(e.g.,Apple T
alk) and convertsit
into apacket format for another
protocol(e.g.,TCP/IP).

104
/ 25
A gateway
SNA network (IBM)
Netware network (Novell)

105
/ 25
Bridges
Divide alarge network into smaller segment
It filters the traffic . It contains logic(Bridge
table) that allows them to keep the traffic for
each segment separate.
Ie) Isolating and controlling the link problems
(e.g.congestion)
Bridges have look-up table that contains physical
address of every station connected to it.

When aframeentersabridge,itchecks theaddressof
thedestinationand forward thenewcopyonlyto the
segmentto whichtheaddresswhich belongs

Types
Simple
Multiport
Transparent
Remote
Source routing

Simple Bridge
It is aless expensive type of bridge
It links 2 segments (LANS) and lists the
address of all the stations in table
included in each of them.
Here address must be entered manually.
The table is modified when stations are
added and removed.

Multiport Bridge
It is used to connect more than two LANS.
So the bridge has 3 tables.
Here address must be entered manually
Transparent Bridge:
A transparent or learning bridge builds its table of
station on its own (automatically).
The table is empty when it is installed, it builds its table
when it encounters the packet for transmission. It
uses the source address for building table.
It identifies the changes and update the table when
system moved from one station to another

Cont.,
Bridges are normally installed
redundantly,that is two LANS may be
connected by more than one bridge.in
this cases they maycreate a loop.
So packet may go round and round,It can
be avoided by algorithms like
Spannig tree algorithm
Source routing

Data Communications and Computer Networks
Remote Bridges
A remote bridge is capable of passing a data frame
from one local area network to another when the two
LANs are separated by a long distance and there is a
wide area network connecting the two LANs.
A remote bridge takes the frame before it leaves the
first LAN and encapsulates the WAN headers and
trailers.
When the packet arrives at the destination remote
bridge, that bridge removes the WAN headers and
trailers leaving the original frame.

Data Communications and Computer Networks
Switches
A switch is a combination of a hub and a bridge (multi-
port bridge).
It can interconnect two or more workstations, but like a
bridge, it observes traffic flow and learns.
When a frame arrives at a switch, the switch examines the
destination address and forwards the frame out the one
necessary connection.
Workstations that connect to a hub are on a shared
segment.
Workstations that connect to a switch are on a switched
segment.

LAN/WLANWorld
LANs provide connectivity for interconnecting
computing resources atthe local levels of an
organization
Wired LANs
Limitations because of physical,hard-
wired infrastructure
Wireless LANs provide
Flexibility
Portability
Mobility
Ease of Installation

Wireless LAN Applications
Medical Professionals
Education
Temporary Situations
Airlines
Security Staff
Emergency Centers

IEEE 802.11 Wireless LAN
Standard
In response to lacking standards, IEEE
developed the first internationally
recognized wireless LAN standard IEEE
802.11
IEEE published 802.11 in 1997, after seven
years of work
Scope of IEEE 802.11 is limited to Physical
and Data Link Layers.

Benefitsof802.11 Standard
Appliance Interoperability
Fast Product Development
Stable Future Migration
Price Reductions
The 802.11 standard takes into account the
following significant differences between
wireless and wired LANs:
Power Management
Security
Bandwidth

WLAN Topology
Ad-Hoc Network
The BSS without an AP is a stand-alone network and cannot send data to otherBSSs.
they can locate one another and agree to be part of a BSS.

WLAN Topology
Infrastructure
EX: cellular network if we consider each BSS to be a cell and
each AP to be a basestation.

StationTypes
IEEE 802.11 defines three types of stations
based on their mobility in awireless LAN:
no-transition
A station is either stationary (not moving) or moving only inside a BSS
BSS-transition
station can move from one BSS to another
, but the movement is confined
inside one ESS.
and ESS-transition mobility
.
A station can move from one ESS to another

collisionavoidanceCSMAICA
network allocation vector (NAV) used to
avoid collision.
RTS frame includes the duration of time that it needs to occupy
the channel.
stations affected by this transmission create a timer called (NAV)
the network allocation vector (NAV) shows the time must pass
before these stations allowed to check the channel for idleness.
there is no mechanism for collision detection,ifthe
sender has not received aCTS frame from the receiver
,
assumes there has been acollision ,thesender tries
again.

BLUETOOTH
Bluetooth is a wireless LAN technology
designed to connect devices of different functions
such as telephones, notebooks, computers,
cameras, printers, coffee makers, and so on. A
Bluetooth LAN is an ad hoc network, which
means that the network is formed spontaneously.
Bluetooth defines two types of networks:
piconet and scatternet.

Piconet
A Bluetooth network is calledapiconet, or asmall net.
It can haveup to eight stations, one of which is called the master; the rest
are called slaves.
Maximum of seven slaves. Only one master
.
Slaves synchronize their clocks and hopping sequence with the master
.
But anadditionaleight slaves canstay in parked state,which means they
canbe synchronized with the master but cannot takepart in
communication until it is moved from the parked state.

Scatternet
Piconets canbecombined to form what is calleda
scatternet.
A slavestation inone piconet canbecome the
master in
another piconet.
Bluetooth devices hasabuilt-inshort-range radio
transmitter.

Bluetooth layers
Radio Layer: Roughly equivalent to physical layerof the Internet model.
Physical links canbe synchronous or asynchronous.
Uses Frequency-hopping spread spectrum [Changing frequencyof usage].
Changes it modulation frequency1600 times per second.
Uses frequencyshift keying (FSK )with Gaussian bandwidth filtering to
transform bits to a signal.
Baseband layer: Roughly equivalent to MAC sublayer in LANs. Access is
usingTime Division (Time slots).
Length of time slot = dwell time = 625 microsec. So, during one frequency
,a
sender sends aframe to aslave,or aslave sends aframe to themaster
.
Time division duplexing TDMA (TDD-TDMA) is akind of half-duplex
communication in which the slave and receiver send and receive data,but
not atthe same time (half-duplex). However
, the communication for each
direction uses different hops, like walkie-talkies.

PhysicalLinks
Synchronous connection-oriented (SCO)
Latency is important than integrity
.
Transmission using slots.
No retransmission.
Asynchronous connectionless link (ACL)
Integrity is important than latency
.
Does like multiple-slave communication.
Retransmission is done.
L2CAP (Logical Link Control andAdaptationProtocol)
Equivalent to LLC sublayer in LANs.
Used for data exchange on ACL Link. SCO channels do not use L2CAP
.
Frame format has 16-bit length [Size of data coming from upper layer in bytes],
channel ID, data and control.
Can do Multiplexing, segmentation and Reassembly, QoS [with no QoS, best-
effort delivery is provided] and Group mangement [Can do like multicast group,
using some kind of logical addresses].

Data Link layer Presentation form Behrouz and furozen

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